The present invention relates to a UV-protected electrochromic device.
Electrochromic devices are already known, for example, from D. Theis in Ullmann""s Encyclopedia of Industrial Chemistry, Vol. A 8, p. 622, Verlag, Chemie 1987 and WO-A 94/23333. A distinction is made between two basic types:
Type 1: all-over electrochromic device.
Type 2: electrochromic display devices with structured electrodes.
Type 1 is used, for example, for electrically darkenable window panes or electrically shadable car mirrors. Such devices are known, for example, from U.S. Pat. No. 4,902,108.
Type 2 is employed in the case of segmented and matrix displays. Such display devices have been proposed, for example, in German Patent Application P 196 31 728. Devices of this kind can be viewed transmissively or, given a mirror coating, reflectively.
WO-A 94/23333 compares electrochromic materials of various construction which, however, are not used as display devices:
Construction a: The electrochromic substances are fixed as a film or coat on the electrodes (cf. Ullmann, see above).
Construction b: The electrochromic substances are deposited a,. a laver on the electrodes by the redox process (cf. Ullmann, see above).
Construction c: The electrochromic substances remain permanently in solution.
For construction a) the best-known electrochromic material is the tungsten oxide/palladium hydride pairing.
For construction b), viologens have been described as electrochromic substances. These devices are not self-erasing, and so the generated image remains after the current has been switched off and can only be erased again by reversing the polarity of the voltage. Such devices are not particularly stable and do not allow a high number of switching cycles.
Cellsxe2x80x94and especially those constructed with tungsten oxide/palladium hydridexe2x80x94cannot be operated in transmitted light but only reflectively, owing to light scattering at these electrochromic layers.
Elektrokhimiya, 13, 32-37 (1977), 13, 404-408, 14, 319-322 (1978), U.S. Pat. No. 4,902,108 and U.S. Pat. No. 5,140,455 disclose an electrochromic system of the last-mentioned construction c). An electrochromic cell which is constructed from conductively coated glass plates contains a solution of a pairing of electrochromic substances in an inert solvent.
As a pairing (couple) of electrochromic substances use is made of one electrochemical reversibly reducible substance and one reversibly oxidizable substance. Both are colourless or only slightly coloured in the ground state. Under the influence of an electrical voltage one substance is reduced and the other oxidized, with both becoming coloured. After the voltage has been switched off, the ground state of both substances is formed again, this being accompanied by disappearance or fading of the colour. 
From U.S. Pat. No. 4,902,108 it is known that suitable redox couples are those where the reducible substance possesses at least two chemically reversible reduction waves in the cyclic voltammogram and the oxidizable substance, accordingly, possesses at least two chemically reversible oxidation waves.
According to WO-A 94/23333, however, such solution systems of construction c have serious disadvantages.
The diffusion of the electrochromic substances in the solution causes imprecise colour boundaries and requires high current consumption to maintain the coloured state, since the coloured substances are permanently broken down by recombination and reaction at the respectively opposite electrode.
Nevertheless, various applications have been described for such electrochromic cells of construction c. For example, they may be configured as car rearview mirrors which when travelling at night can be darkened by applying a voltage and so prevent dazzling by the headlights of following vehicles (cf. e.g. U.S. Pat. No. 3,280,701, U.S. Pat. No. 4,902,108, EP-A 0 435 689). In addition, such cells can also be employed in window panes or car sunroofs, where following application of a voltage they shade out the sunlight. Likewise described is the use of such devices as electrochromic display devicesxe2x80x94for example in segmented or matrix displays with structured electrodes (German Patent Application P 196 31 728).
The electrochromic cells normally consist of a pair of glass plates of which one is mirrored in the case of the car mirror. One side of these plates is coated over its surface with a transparent, electrically conductive coat of, for example, indium tin oxide (ITO), and in the case of the display devices this conductive coating is divided into segments which are electrically separate from one another and are contacted individually. These plates are then used to construct a cell by joining them via a sealing ring with their electrically conductively coated sides facing one another, to form a cell. This cell is then filled by an aperture with an electrochromic fluid. and the cell is sealed tightly. The two plates are connected to a voltage source by way of the ITO coats.
The above-described electrochromic devices are generally sensitive to light, especially UV light. Consequently, in U.S. Pat. No. 5,280,380, for example, electrochromic devices have been described which comprise UV stabilizers.
The UV light absorbers used to date have mostly been organic compounds which have a molecular absorption band in the relevant wavelength range and do not absorb in the visible spectral range. A disadvantage of these compounds is that they must be dissolved, in some cases in high concentrations, in the electrochromic solution or in a polymer layer applied to one of the two plates. Frequently, however, the solubility in these media is limited and so, consequently, is the effectiveness of the UV absorber. In addition, they may fade on exposure to light and/or may be washed out or evaporate from the polymeric substrate.
It is also known that inorganic solid-state materials can absorb UV light.
Depending on the size and choice of material, these inorganic particles may scatter and/or absorb regions of the harmful UV light. Preference should be given in this context to absorption over scattering of light, since scattered photons, especially when the particles are incorporated into the material that is to be protected, may go on to damage this material. In addition, too great a scattered component of the light leads to clouding of the material that is to be protected. From Absorption and Scattering of Light by Small Particles, C. F. Bohren, D. R. Huffman, pp. 93 to 104 and 130 to 141, 1983 it is known that as the size of the particles decreases their absorbency for light is higher than their capacity to scatter light. For a transparent UV light absorber, therefore, only very small particles are suitable. In order to ensure that the UV light absorber is also colourless, the material of the particles must possess an absorption edge in the wavelength range between about 300 nm and 400 nm. According to WO-A 93/06164, materials suitable for such an effect are those having a band gap between 2.8 eV and 4.1 eV, corresponding to a wavelength range of between 303 nm and 445 nm. Materials from this class which have already been used for this purpose include TiO2, ZnO, CeO2 and SiC: see for example WO-A 93/06164, WO-A 95/09895 and WO-A 92/21315.
Consequently, the object was to provide UV absorbers which do not have the disadvantages known from the prior art and which are highly suitable for the UV protection of electrochromic cells.
It has now been found that the above-described electrochromic devices can be effectively protected against destruction by UV light by means of nanoparticles.
The invention accordingly provides an electrochromic device which is protected by nanoparticles against UV light.
The invention provides preferably an electrochromic device consisting of two plates or films of which at least one is transparent and which are provided on the facing sides with a conductive layer, at least one conductive layer being transparent, and are joined to one another with a sealing ring, the plates or films and the sealing ring defining a volume within which there is an electrochromic medium, characterized in that the electrochromic device is protected by nanoparticles against UV light.